This invention pertains generally to radar transmitters, and particularly to an improved frequency-agile transmitter wherein non-common mode noise is significantly reduced.
In the field of radar countermeasures a requirement for transmitters having wider bandwidths, frequency agility and lower noise has been increasingly evident. To meet the need for frequency agility, and at the same time provide waveform diversity, transmitter designs have been developed wherein the microwave source comprises a combination of two, or more, separate microwave generators whose output signals are combined and amplified in relatively broadband amplifiers prior to being transmitted. While a wide operating bandwidth may be obtained with any of such transmitter designs, spurious signals and increased noise levels are also experienced. Such spurious signals are intermodulation products that occur when the output signals of two, or more, separate microwave generators are combined. Those spurious signals falling within the wide operating bandwidth of the transmitter cannot easily be eliminated by filtering. As a result, then, spurious signals detract from the purity of the transmitted waveform.
With respect to the problem of increased transmitter noise, a distinction must be made between "common mode" noise, i.e. noise present on both the transmitted and local oscillators signals, and "non-common mode" noise, i.e., noise generated within the transmitter amplifiers. While common mode noise may be partially cancelled, the wide instantaneous bandwidths required for frequency agility usually result in amplifiers having increased non-common mode noise levels vis-a-vis those used for narrow band operation.